1. Field of the Invention
The present invention relates to a control device for a hybrid four-wheel-drive vehicle wherein the front or rear wheels are driven by an engine, and the others are driven by an electric motor.
2. Description of the Related Art
As an example of hybrid four-wheel-drive vehicles, a hybrid four-wheel-drive vehicle wherein the front wheels are driven by an engine, and the rear wheels are driven by an electric motor is known, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-63392, paragraph 0004, for example (which will be referred to as “Patent Document 1” hereafter). With the four-wheel-drive vehicle disclosed in this Patent Document 1, in the event that the front wheels slip or spin (which will be generically referred to as “slipping” hereafter) during acceleration, the engine drives a generator serving as a power supply for an electric motor for driving the rear wheel. With such a configuration, a part of the engine output is used for driving the generator for supplying electric power to the electric motor, thereby reducing the driving force of the front wheels so as to prevent slipping of the front wheels. Furthermore, even in the event that such operation is insufficient for suppressing slipping of the front wheel, the system reduces the engine output so as to suppress slipping of the front wheels in a sure manner.
However, with the control method disclosed in Patent Document 1, in the event that the engine-driven wheels slip during acceleration, the engine output is reduced so as to suppress slipping, leading to reduced rotational speed of the generator driven by the engine, and reduced torque for driving the generator, resulting in reduced generated electric power. This leads to reduced electric-current supply to the electric motor, resulting in insufficient torque of the electric motor. Furthermore, in the event that the induced voltage of the electric motor exceeds the voltage generated by the generator, the generator cannot supply current to the electric motor, may lead to a problem that the electric motor cannot be driven in such cases. With the aforementioned configuration further including a mechanism wherein an electricity storage device such as a storage battery or the like supplies electric power to the electric motor in such a case, the electric motor generates sufficient torque even in such a case. However, with such a configuration, continuous driving may reduce the electricity stored in the electricity storage device, may lead to a problem that the electric motor cannot be driven in such a case. As described above, the control methods according to the conventional technique have a problem that in the event that the engine-driven wheels slip during acceleration, the electric-motor-driven wheels may not be driven with sufficient driving force, leading to reduction of the acceleration performance of the vehicle.